Preface to the Special Issue:Hydrogen Embrittlement and Storage

There is no doubt that hydrogen(H)is becoming one of the most promising sustainable energy carriers which can release at least partially the current strong reliance on fossil fuel-based energy and the resulting environmental burden.

Effects of post annealing on the microstructure,precipitation behavior,and mechanical property of a(CoCrNi)_(94)Al_(3)Ti_(3)medium-entropy alloy fabricated by laser powder bed fusion

The additive manufacturing of multi-principal element alloys has remarkable potential for industrial ap-plications.In this study,a(CoCrNi)_(94)Al_(3)Ti_(3)medium-entropy alloy(MEA)with adequate strength-ductility synergy was prepared via laser powder bed fusion.The microstructural evolution,mechanical property,and deformation mechanisms of the MEA were investigated after post annealing for a short period(0.5 h)at a temperature range of 773-1373 K using various microstructural characterization techniques and quantitative analysis.The static recrystallization temperature of the(CoCrNi)_(94)Al_(3)Ti_(3)MEA ranged from 973 to 1073 K.The average grain size first decreased and then increased,while the dislocation den-sity persistently decreased and texture gradually weakened with increasing annealing temperature.Cr-richσ-phase precipitates formed after 1073 K and then gradually dissolved at 1373 K,while Ni,Al,and Ti elements were aggregated to form a small amount of fine L1_(2)coherent precipitates with an aver-age diameter of approximately 70 nm at 1373 K.The evolution of the dislocation density,grain size,and precipitates significantly influenced the propensity of deformation twins and stacking faults,which consequently affected the strain hardening behavior and mechanical properties.The quantitative calcu-lation of strengthening mechanisms showed that dislocation strengthening played a dominant role at annealing temperatures below 1073 K,and it significantly weakened at 1373 K.Precipitation and grain boundary strengthening both markedly increased owing to the formation of precipitation particles and recrystallization-induced grain refinement after annealing at 1073 K.

Enhancing the Hydrogen Embrittlement Resistance of Medium Mn Steels by Designing Metastable Austenite with a Compositional Core-shell Structure

Deformation-induced martensite transformation from metastable retained austenite is one of the most efficient strain-hardening mechanisms contributing to the enhancement of strength-ductility synergy in advanced high-strength steels.However,the hard transformation product(often-martensite)and the H redistribution associated with phase transformation essentially decrease materials’resistance to hydrogen embrittlement.To solve this fundamental conflict,we introduce a new microstructure architecting strategy based on an accurately design of core–shell compositional distribution inside the austenite phase.We employed this approach in a typical medium Mn steel(8 wt.%Mn)with an ultrafine grained austenite-ferrite microstructure.We produced a high Mn content(15–16 wt.%)in the austenite shell region and a low Mn content(~12 wt.%)in the core region,through a thermodynamics-guided two-step austenite reversion treatment.During room-temperature deformation,the austenite core transforms continuously starting from a low strain,providing a high and persistent strain-hardening rate.The transformation of Mn-rich austenite shell,on the other hand,occurs only at the latest regime of the deformation,thus effectively inhibiting the nucleation of H-induced cracks at ferrite/deformation-induced martensite interfaces as well as suppressing their growth and percolation.This step-wise transformation,tailored directly targeted to protect the hydrogen-sensitive microstructure defects(interfaces),results in a significantly enhanced hydrogen embrittlement resistance without sacrificing the mechanical performance in hydrogen-free condition.The design of compositional core–shell structure is expected to be applicable to,at least,other multiphase advanced high-strength steels containing metastable austenite.

Current Challenges and Opportunities Toward Understanding Hydrogen Embrittlement Mechanisms in Advanced High-Strength Steels: A Review

Hydrogen embrittlement(HE)is one of the most dangerous yet most elusive embrittlement problems in metallic materials.Advanced high-strength steels(AHSS)are particularly prone to HE,as evidenced by the serious degradation of their load-bearing capacity with the presence of typically only a few parts-per-million H.This strongly impedes their further development and application and could set an abrupt halt for the weight reduction strategies pursued globally in the automotive industry.It is thus important to understand the HE mechanisms in this material class,in order to develop effective H-resistant strategies.Here,we review the related research in this field,with the purpose to highlight the recent progress,and more importantly,the current challenges toward understanding the fundamental HE mechanisms in modern AHSS.The review starts with a brief introduction of current HE models,followed by an overview of the state-of-the-art micromechanical testing techniques dedicated for HE study.Finally,the reported HE phenomena in different types of AHSS are critically reviewed.Focuses are particularly placed on two representative multiphase steels,i.e.,ferrite–martensite dual-phase steels and ferrite–austenite medium-Mn steels,with the aim to highlight the multiple dimensions of complexity of HE mechanisms in complex AHSS.Based on this,open scientific questions and the critical challenges in this field are discussed to guide future research efforts.

Evaluation of hydrogen effect on the fatigue crack growth behavior of medium-Mn steels via in-situ hydrogen plasma charging in an environmental scanning electron microscope

Fatigue crack growth(FCG)tests were conducted on a medium-Mn steel annealed at two intercritical annealing temperatures,resulting in different austenite(γ)to fe rrite(α)phase fractions and differentγ(meta-)stabilities.Novel in-situ hydrogen plasma charging was combined with in-situ cyclic loading in an environmental scanning electron microscope(ESEM).The in-situ hydrogen plasma cha rging increased the fatigue crack growth rate(FCGR)by up to two times in comparison with the reference tests in vacuum.Fractographic investigations showed a brittle-like crack growth or boundary cracking manner in the hydrogen environment while a ductile transgranular manner in vacuum.For both materials,the plastic deformation zone showed a reduced size along the hydrogen-influenced fracture path in comparison with that in vacuum.The difference in the hydrogen-assisted FCG of the medium-Mn steel with different microstructures was explained in terms of phase fraction,phase stability,yielding strength and hydrogen distribution.This refined study can help to understand the FCG mechanism without or with hydrogen under in-situ hydrogen charging conditions and can provide some insights from the applications point of view.